Solar energy is one of the new energies that are most promising and most likely to meet continually increased demand for energy in future social development, and has characteristics such as unlimited reserves, wide distribution, clean utilization and the economical efficiency. The solar thermal power generation has some characteristics, for example, better adaptability to power grid load, high photoelectric conversion efficiency, scale effect with ease, environment-friendly manufacturing process of consumptive material, better adjustability of the electric power, and so on. Thus, the solar thermal power generation is an important development direction of utilization of solar power generation in the future.
The basic technical idea of the solar thermal power generation is that: the sunlight is converged through a heat collector to increase the energy density of light energy; the collected light energy is absorbed by a heat absorbing device and converted into heat energy; the heat energy is transferred to working medium to increase the internal energy of the working medium; and then the internal energy in the working medium is converted into mechanical energy through a heat engine and a generator is driven so that the mechanical energy is further converted into electric energy to be output. In the whole process of energy conversion, converting the heat energy into the mechanical energy is the most critical aspect.
Currently, there are mainly three kinds of the heat engine that are applicable to the solar thermal power generation system, i.e., the steam turbine based on Rankine cycle, the Stirling engine based on Stirling cycle and the small-scale gas turbine based on Brayton cycle. Specifically, the steam turbine can use hydrocarbons (halogenated hydrocarbons) or water having low boiling point and good heat stability as the working medium. However, because the temperature that the working medium can withstand is low, the heat efficiency is low. The steam turbine is generally used in a slot-type power generation system with low the heat collection temperature. The Stirling engine uses hydrogen or helium working medium which has the dynamic seal pressure up to 15 Mp or more when working, so that the working reliability, stability and lifetime is limited to some extent. The small-scale gas turbine can directly use air as the working medium. That is, air is compressed by a compressor, then absorbs heat and is heated up in a working medium heating device, and then goes into a turbine for expanding and doing work; and the mechanical work in turn drives the compressor and the generator for outputting current. The small-scale gas turbine is simple in the design, has no demanding seal conditions, directly obtains and discharges the working medium from and into atmosphere, and has better reliability and stability.
However, for using the small-scale gas turbine as the heat engine for solar thermal power generation equipment, there are following several problems to overcome in addition to difficulty in designing impellers of the compressor and the turbine with high efficiency as well as high speed generator:
1) start-up performance of the system: because the turbine and the compressor are coupled to each other, after the compressor drives the high-pressure airflow into the heat collector, the heat generated by the heat collector can be absorbed by the airflow, and the formed high-temperature and high-pressure airflow can pass through the turbine to output mechanical work and to drive the compressor and the generator. Thus, when the system is actuated, an additional starting device is required to give an initial rotating speed to the compressor. In this way, the whole system can be actuated smoothly, resulting in a more complex structure of the thermoelectric conversion device.
2) lifetime and reliability of the high speed generator: because the operating rotating speed of the small-scale gas turbine is up to 100000 to 200000 r/min, cooling requirement of the generator is extremely demanding. It is necessary to provide a good solution to cooling, otherwise the lifetime and reliability of the generator will be affected.
3) operation stability and robustness of the system: when the high-temperature air going into the turbine air inlet deviates from the designed working temperature and pressure of the turbine due to fluctuation in solar radiation and so on, the rotating speed of the turbine impeller will significantly fluctuate, resulting in a fluctuation in the rotating speed of the turbine impeller, and the flow and pressure of the air going into the working medium heating device will fluctuate as well, thus further leading to fluctuation in the rotating speed of the turbine impeller, and causing loss of stability of the system.